Electroless plating



May 7, 1957 c. G. CHAMBERS ETAL ELECTROLESS F'LATING Filed Sept. 1'7.1953 HEFT EXK'HNER mir/Nq United ELECTROLESS PLATING ApplicationSeptember 17, 1953, Serial No. 380,858

9 Claims. (Cl. 117-102) This invention relates to improvements in thedeposition of metal from chemical reduction type plating solutions.

It is known that nickel and other metals can be deposited without theuse of electrical current by immersing an article to be coated into anaqueous solution of a nickel or other metal salt and a reducing agentsuch as sodium hypophosphite or potassium hypophosphite. There are twotypes of such plating solutions, namely, alkaline and acid, of which theacid solution is generally better adapted for commercial platingoperations. The plating by chemical reduction is usually conducted at anelevated temperature, typically about 90 C.

One' of the diicul'ties heretofore encountered in the commercialadaptation of chemical reduction plating has been the inability topreclude deposition of metal on the plating equipment with the resultthat in many instances an undesirable metallic deposit is producedthroughout the plating equipment. Not only does such a deposit depletethe metal content of the plating solution but it also impairs heattransfer and acts as a catalyst increasing the undesired deposition ofmetal.

The exact nature of the chemical plating reaction is not clearlyunderstood at present. However, it has been observed that certainmetals, such as lead, copper, brass or platinum which would notordinarily be coated by the oxidation-reduction plating reaction, dobecome susceptible to being coated when, for example, momentarilybrought in contact with aluminum or other reaction initiating material.Inasmuch as the exact nature of such an initiating reaction is notclearly understood the failure to control lthe deposition of nickel andother metals deposited by chemical reduction methods in certain cases isa serious problem even though the plating equipment is formed from amaterial which theoretically should not become coated during the platingreaction. The problem, of course, is even more serious in large scalereduction plating operations which employ plating tanks, piping andVother equipment formed of metals, such as iron, steel, nickel, aluminumand alloys of these metals, which are readily coated with the metalbeing deposited.

A principal object of the present invention is to provide a chemicalreduction plating process in which the aforementioned difticulties areminimized. A further object is the provision of an improved chemicalreduction plating process. A still further object is the provision of animproved plating operation in which maintenance of proper solutionconcentration is facilitated while effectively controlling the metallicdeposition. These and other objects and advantages of the invention willmore fully appear from the following description taken in connectionwith the accompanying drawing which illustrates in schematic formapparatus for the operation of a chemical reduction plating process inaccordance with the present invention.

Generally, we have discovered that undesirable metallic depositionthroughout the plating equipment can effectively be minimized andprecluded entirely in many arent M 2,791,516 ce Patented May 7, 1957applications by constantly recirculating the plating solution and byquench cooling a portion of this solution from the high bath temperaturerequired for optimum plating to a temperature at which little or noplating occurs and thereafter recirculating the cooled solution into theplating tank. By proper control, using this technique, sufficientplating solution is quenched to preclude metallic deposition throughoutthe equipment; yet an excessive amount of heat energy is not wasted linso doing.

In the drawing the single figure is a schematic diagram of apparatusused in performing the plating process.

For the continuous operation of a plating solution, quench cooled inaccordance with the invention, the maintenance of proper solutionconcentration is contemplated by continuously or intermittentlyintroducing into the circulating plating solution the required amounts.of bath ingredients. This addition preferably is made by charging alfilter, used in chemical reduction plating operations, with properamounts of bath additives to maintain the desired concentration.

Referring now more particularly to the accompanying drawing, whichillustrates preferred apparatus comprising at least one plating tank, aheat exchanger, a filter and a pump connected in series, and controlvalves for carrying out the practice of the invention, articles whichare to be coated with nickel or other metal are immersed in a reductionplating solution in plating tanks indicated at 1t) and 12. Although twotanks are illustrated, it will be understood, of course, that one or anynumber of tanks may be employed, a plurality of tanks merely beingconnected in parallel, each in series with the heat exchanger, filter,and pump.

For proper deposition of nickel by chemical reduction plating, thetemperature of the plating solution should be relatively high. Moreover,as mentioned above, in order to remove harmful ingredients eitherprecipitated or mechanically dragged into the bath, the solution iscontinuously filtered. One of the most difficult problems heretoforeencountered has been Ithat of precluding or minimizing the continueddeposition of nickel or other metal as the hot plating solution ispassed through the filter and other auxiliary equipment.

The practice of the present invention comtemplates circulating platingsolution, heated to the desired temperature in a conventional manner asby steam coils (not shown), through plating tanks 10 and l2, a heatexchanger 14, a fluid pump i6 and a filter 18 connected in series withsaid tanks, and withdrawing hot used plating solution from the platingtanks 10 and l2 through pipes 2i) and 22, the ow being regulated byvalves 24 and 26, and rapidly quench cooling the solution to atemperature at which plating does not occur. This is accomplished in theapparatus illustrated in the drawing by recycling cooled platingsolution from the heat exchanger, having a cooling fluid inlet 28 andoutlet 30, by means of a pump 16, back through pipes 32, 34, 36 and 38,control valves 40, 42 and 58, to the heat exchanger where it mixes withthe hot plating solution being withdrawn from the plating tanks throughpipes 20 and 22 and serves to immediately quench the hot platingsolution to a lower temperature. ln operation, the plating solution fromthe heat exchanger 14 may be continuously filtered by passing a portionof it through pipe i4 and filter 18 and recycling the remainder throughthe heat exchanger 14. lf desired, a portion of the circulating bath canbe simultaneously recycled to an addition tank 60 through pipe 46 andvalve 48.

Although the quantities of solutions and flow rates thereof throughpipes generally will be dictated primarily by the metal being deposited,the heat content of the solutions, the temperatures involved and theparticular bath composition, in a particular instance excellent resultshave been obtained by recycling approximately three times the volume ofhot plating solution continuously being removed from the plating tank.The temperature of the hot plating solution removed from the platingtanks, typically about 195 F., is thus quickly reduced to about 160 F.by mixing therewith three times its volume of cooled recirculatedsolution having a temperature of about 145 F. The 160 F. temperature isthereafter reduced to 145 F. by passing the solution through the heatexchanger, a portion of this etiluent being passed to the filter andthence to the plating tank through valves 5t), 52 and pipes 54 and 56while the remainder is recycled by the pump 16 to the heat exchangerthrough pipe 38 and control valve 53 as described above.

The invention contemplates the use, preferably in combination with theaforementioned quench cooling of the hot used plating solution, theoriginal bath makeup use of, or bath maintenance by, one or morewater-insoluble salts of the metal to be deposited together with theappropriate reducing agent and/ or other bath additives. In depositingnickel, for example, the proper plating solution concentrationspreferably are maintained by addition of water-insoluble nickel salts.In practice, metal salts, as well as other bath ingredients, are addedto the bath through an addition tank 60 where the resulting slurry, orsolution in case it is desired to employ water-soluble salts, is pumpedto the filter 18 and thence either back to the addition tank 60, havingan outlet pipe 76 and control valve 78, or to the plating tanks 10 and12.

Depending upon the particular type of lter employed it is desirable inmany instances to employ a lter aid, such as linely divided asbestos ordiatomaceous earth, in order to form a flterable cake. Such a filter aidcan be added to the mixture in the addition tank 60 and thence pumped tothe lter. In the drawing, the filter 18 is provided with a gas Outlet 62and control valve 61 to permit discharge of carbon dioxide or othergases formed during gradual dissolution of bath additives into theplating solution. To facilitate cleaning the lter a compressed airsource (not shown) can be discharged through the filter 1S by means ofvalve 64 and inlet pipe 66, valve 68 permitting draining the apparatuswhen desired.

When water-insoluble salts of the metal to be deposited are used, aslurry is formed in the addition tank 60, valves 40 and 74 are eitherclosed or adjusted to permit the desired liow and the slurry is pumpedthrough valve 78 and pipe 76 to the lter 18 where it deposits over thefilter media, formed typically of heat and chemical resistant cloth oriiber. If desired to build up a thicker coating on the filter media,valve 72 and valve 74 are closed and the solution is recycled back tothe addition tank 60 through pipe 46 and valve 48. The plating solutionbeing passed through the lter 18 gradually dissolves the metallic saltsas the metal already in solution is depleted in the plating operation.Hence the procedure provides an extremely cicient method of controllingthe plating operation and maintaining the proper bath concentration.

The present invention is particularly well adapted for use in thechemical reduction deposition of nickel. Typical of a nickel reductionplating bath which may be regulated in operation by the practice of thepresent invention is the following bath composition employingwater-soluble nickel salts where the quantities expressed are per literof water:

Nickel chloride grams 30 Sodium hypophosphite do l Glycollic acid (70%)-milliliters 35 Sodium hydroxide -grams 15 It will be understood ofcourse that the above bath composition is intended only as anillustration of a typical reduction plating bath and that the presentinvention is applicable to a wide range of nickel bath compositions. Forexample, plating baths containing about 5 to 50 grams per liter of awater-soluble nickel salt such as nickel chloride, nickel acetate,nickel sulfate, etc.; approximately 5 to 100 grams per liter of areducing agent, such as hypophosphorous acid, sodium hypophosphite orother soluble hypophosphites; and about 15 to 100 grams per liter of abutter such as glycollic acid, sodium hydroxy acetate, sodium acetate,sodium citrate, etc. and/or other bath additives can similarly bereplenished in accordance with the present invention.

It is to be understood that the present invention also contemplates theformation of an improved nickel chemical reduction plating bathutilizing a water-insoluble nickel salt as an initial ingredient. Such abath comprises an aqueous mixture of a reducing agent and aWaterinsoluble nickel salt selected from the group consisting of oxynickel salts such as nickel monoxide, nickel peroxide, nickelsesquioxide, nickelous nickelic oxide and nickelous superoxide; hydroxynickel salts such as nickelic hydroxide and nickelous hydroxide; andnickel carbonates such as nickel carbonate and basic nickel carbonate.In most instances it is desirable to additionally 'employ a butleragent. A preferred bath of this type comprises nickel carbonate, sodiumhypophosphite, and glycolic acid.

A bath of the above-mentioned type can be produced by dissolving thedesired water-insoluble nickel salt or salts in glycollic acid andthereafter adding the reducing agent. lf another type buffer is used,for example, sodium acetate, sodium citrate or sodium hydroxy acetate,the nickel salt may first be dissolved in hypophosphorus or phosphorusacid, enough acid being used to obtain the desired initial bath pH.

The following is an example of one reduction plating bath utilizing awater-insoluble nickel salt as an initial ingredient. The quantitiesexpressed are per liter of water.

Nickel carbonate gramsn 15 Glycollic acid (70%) milliliters.- 35 Sodiumhypophosphite grams l0 Preferred pH 5.0

A satisfactory range of ingredients comprises a mixture of about 5 to100 grams per liter of a water-insoluble nickel salt, approximately 15to 100 grams per liter of a butter such as glycollic acid, sodiumhydroxy acetate, sodium acetate, sodium citrate, etc., and about 5 to100 grams per liter or reducing agent such as sodium hypophosphite,hypophosphorous acid, or other hypophosphites.

We have discovered that excellent results are obtained when the platingtanks, typically of steel or the like, are coated on the interior withporcelain or a heat resistant imprevious organic coating, a particularexample being the commercially available phenolic coating known asHeresite, a product of the Heresite & Chemical Co. Supenior results havebeen obtained when the equipment coming in contact with the hot platingsolution is formed of tantalum which is extremely resistant tobeing-coated by chemical reduction plating.

It is to be understood that, although the invention has been describedwith specific reference to particular embodiments thereof, it is not tobe so limited since changes and alterations therein may be made whichare within the full intended scope of this invention as defined by theappended claims.

What is claimed is:

l. A method of depositing a metal from a chemical reduction type platingbath, said methodl comprising the steps of immersing an article to becoated into said plating bath which is maintained at an elevatedtemperature and is continuously recirculated through a filter, saidplating bath being cooled just prior to passage through said lter bymixture therewith of cooled plating solution.

2. In the deposition of nickel from a hot chemical reduction platingbath of the type comprising an aqueous solution of at least one nickelsalt and a reducing agent in which said solution is continuouslyrecirculated through a ilter to thereby remove harmful impurities, theimprovement which consists of precluding nickel deposition throughoutthe ilter and associated apparatus as the plating solution circulatestherethrough by passing the hot plating solution through a heatexchanger to cause said solution to cool, a portion of the cool platingsolution from said heat exchanger being mixed with the hot platingsolution as it is withdrawn from said both to thereby rapidly reduce thesolution temperature below that temperature at which plating occurs.

3. Apparatus for chemical reduction plating operations, said apparatuscomprising .a plating tank, a heat exchanger and a filter connected inseries by conduits to permit fluid flow from said tank through said heatexchanger and then through said filter and back to said tank, and meansfor recycling ya portion of the fluid effluent of said heat exchanger tothe inlet of said heat exchanger.

4. Apparatus for chemical reduction plating operations, said apparatuscomprising a container for plating solution, a heat exchanger, a filterand a pump connected in series by conduits for the passage of saidplating solution therebetween and valve means in said conduits torecirculate a portion of the heat exchanger etlluent plating solution tothe inlet of said heat exchanger.

5. Apparatus for carrying out chemical reduction plating operations,said apparatus comprising at least one plating tank, a heat exchanger, afilter and a fluid pump connected in series by conduits to permit fluidow therebetween, said apparatus having control valve means for alternateand simultaneous tiltening and recycling through said heat exchanger ofa portion of said plating solution.

6. Apparatus for carrying out chemical reduction plating operations,said apparatus comprising a container for plating solution and a heatexchanger connected by conduits to permit uid flow therebetween, meanstin said apparatus for maintaining proper solution concentration, andmeans for recycling a portion of the heat exchanger eluent to the inletof the heat exchanger.

7. In apparatus for use in chemical reduction plating operations, saidapparatus comprising at least one container for plating solution, a heatexchanger, a filter and a pump, connected by conduits for series lluidtiow, the improvement which comprises control valve and piping meansadapted to withdraw a portion of cooled plating solution from said heatexchanger and admix it with the plating solution passing from thecontainer to heat exchanger.

8. Apparatus .as set forth in claim 7 in which portions of the apparatusin contact with the plating solution are formed of tantalum.

9. In a process for the deposition of nickel from a chemical reductionplating bath, said bath comprising an aqueous solution of at least onenickel salt and a reducing agent, the improvement which compriseswithdrawing the solution from said bath and passing it through a liltercharged with at least one nickel salt to thereby replenish the nickelsalt in said solution and simultaneously remove solid material from saidsolution, and then returning said solution to said bath, said solutionbeing rapidly cooled just prior to passage through said filter to atemperature below that at which plating occurs.

References Cited in the file of this patent UNITED STATES PATENTS2,009,232 Hood July 23, 1935 2,516,349 Smith July 25, 1950 2,516,884Kyame Aug. 1, 1950 2,658,839 Talnley et al. Nov. l0, 1953

9. IN A PROCESS FOR THE DEPOSITION OF NICKEL FROM A CHEMICAL REDUCTIONPLATING BATH, SAID BATH COMPRISING AN AQUEOUS SOLUTION OF AT LEAST ONENICKEL SALT AN A REDUCING AGENT, THE IMPROVEMENT WHICH COMPRISESWITHDRAWING THE SOLUTION FROM SAID BATH AND PASSING IT THROUGH A FILTERCHARGED WITH AT LEAST ONE NICKEL SALT THEREBY REPLENISH THE NICKEL SALTIN SAID SOLUTION AND SIMULTANEOUSLY REMOVE SOLID MATERIAL FROM SAIDSOLUTION, AND THEN RETURNING SAID SOLUTION TO SAID BATH, SAID SOLUTIONBEING RAPIDLY COOLED JUST PRIOR TO PASSAGE THROUGH SAID FILTER TO ATEMPERATURE BELOW THAT AT WHICH PLATING ACCURS.